1. Field of the Invention
An electric parking brake for a motor vehicle, and more specifically, a parking brake used as an additional service brake for vehicle deceleration.
2. Description of Related Art
In the field of motor vehicles, electric parking brakes are increasingly replacing conventional brake-cable parking brakes. Besides various advantages—like saving space regarding the operating elements and eliminating the brake cables—electrically controlled parking brakes can also be automatically activated and deactivated by an engine control unit independently of operator action and can also carry out other functions—like an automatic anti-roll-back device—during active vehicle operation.
It is known to use the parking brake as an additional service brake in certain situations in which the braking forces applied by the regular service brake are insufficient, e.g., if a technical disruption occurs or an insufficient residual stroke of a hydraulic main brake cylinder.
Electrically supported brake systems, using an electric motor instead of a conventional, vacuum-actuated brake booster, are in the field of service brakes. If the electric braking-force support failed, enough service-brake force through the hydraulic brake system is not always ensured, since with electrically supported brake systems, relatively low-volume tandem main cylinders are often used, which may not be capable of providing enough hydraulic fluid if failure of the electrical support occurs and manual actuation.
Independently thereof, if failure of the braking-force support occurs, the pedal travel and pedal forces increase, which can cause the driver of the vehicle ultimately applying insufficient braking forces. It can be useful to use the electric parking brake to apply supportive service-brake force.
Commonly used electric parking brakes are actuated by brake actuators, mounted directly on the brake disks of the wheels, wherein the actuators selectively press a brake shoe onto the brake disk or release the brake disk with a direct-current commutator motor. The conversion of the motor rotation into a linear motion is usually carried out through a reduction gear and a self-locking worm gear, and so the braking force can be maintained in a currentless state, in accordance with the requirements of a parking brake.
It is common to provide both rear wheels with such an electric-motor brake actuator. It is also possible to equip the front wheels or all wheels with such actuators.
Use of the electric parking brake as a service brake, may require a rapid activation of the parking brake during operation, i.e., the electric motors of the parking brake should build up the braking force with the maximum possible speed or the maximum possible torque.